Magnetic Resonance Imaging (MRI) at high field values is fundamentally advantageous in its high sensitivity and its greater spectroscopic resolution than obtainable for lower field strength. High field MRI also has the capability of functional studies, such as functional brain mapping and various relaxation and diffusion studies. The probe/transmitter, which encases the segment of anatomy under study and constitutes the front end of the MRI system is essential for the realization of these advantages.
A well known problem in MRI systems having probes with large volumes is the prominent and complicated probe-subject interaction which changes the field distribution and resonance frequency of the probe. General compensating adjustments are usually ineffective since the changes in resonance frequency vary significantly among subjects, and is based on factors too numerous to easily factor into general system adjustments. Another problem is the substantial radiation losses of high field coils operating at high frequencies.
For example, MRI imaging at 4 Tesla requires that volume coils maintain efficiency and minimize susceptibility to perturbations introduced by the human body under test. This is so because the wavelength of proton resonance at 4 Tesla in water is approximately 20 cm, which is on the same order as the subject under test. This rough equality in size results in increased perturbation of the RF field pattern from the subject, and the perturbations become far more sensitive to the specific geometry and other characteristics of the subject. The increased perturbations also cause more significant loading frequency shifts. Consequently, it is necessary to be able to tune the coil easily for each subject without upsetting the symmetry of the coil which is necessary for maintaining the uniform of field pattern as well as quadrature drive and reception. Quadrature drive considered highly desirable since the RF power requirement is reduce by a factor of 2 and the signal to noise ratio is increased by a factor of .sqroot.2.
One solution to this problem is a "bird cage" coil having a design based on a lumped-element delay line. This system is proposed in the Journal of Magnetic Resonance 63, pages 622-628 (1985) by an article entitled "An Efficient, Highly Homogeneous Radio Frequency Coil for Whole-Body NMR Imaging at 1.5T", authored by Cecil E. Hayes, William Edelstein, John F. Schenck, Otward Mueller and Matthew Eash. In this article, the use of the "bird cage" coil is studied with respect to using quadrature excitation and reception. The advantage of the arrangement described in this publication is that the current distribution in the coil give a more uniform signal sensitivity. The system of this article appears to be limited to a range of operation of approximately 1.5 Tesla proton or 65 MHz.
Another approach is to provide a tuning mechanism in the coil so that the overall coil resonance frequency can be tuned based upon the characteristics of each subject being tested. Such a system is disclosed in the 1992 Abstract for Scientific Presentations to the Society of Magnetic Resonants in Medicine at the Annual Scientific Meeting conducted in Berlin, Germany on Aug. 14, 1992. This article is entitled "A High Frequency Tuned Resonator for Clinical NMR" and is authored by J. T. Vaughan, J. O. Otu, H. P. Hetherington, P. Noa, and G. M. Pohost. This publication describes a head coil designed for operation at 175 MHz and 4.1 Tesla. The coil is designed to have a transverse virtual ground plane established by coaxial line segments and is configured in the "bird cage" arrangement disclosed in the previously discussed publication. The coaxial line segments are spaced around the periphery of the "bird cage" structure and run between the top and bottom plates. All of the coaxial conductors can be simultaneously tuned to maintain the symmetry necessary for quadrature excitation. With this arrangement, the head coil can be tuned over a limited range of approximately 20 MHz (175 MHz plus/minus 10 MHz) by means of a single screw. Field symmetry and magnitude are not significantly effected by this tuning. However, the unloaded and loaded Q values for the connected resonator average 420 vs. 65.
The conventional art previously described still suffers from the drawback of a low Q factor under high load conditions.